The DCMA was a mistake, as Bandai Namco enlisted the FDS File Defense System to take down all patches that enable debug mode. As DSFix does not do this, it is officially allowed to be uploaded.
The DSFix mod, created by Durante, has come under fire from Dark Souls publisher Bandai Namco.
It's an interception DINPUT8.dll that you place in the same folder as the game executable. It intercepts the game's calls to the DirectX 9 API and changes them as necessary to enable a higher internal rendering resolution and various other features.
DSFix basically improves performance by altering system calls to DirectX 9, to make the game more enjoyable on PC.
Posting to the NeoGaf forum, Durante pasted up the email he received from Dropbox, telling him that his upload of DSFix 2.3 had received the DMCA complaint, and been removed from their servers.
As the game doesn't contain any files created by Bandai Namco, this is a strange thing for them to be claiming against. Other users on the forum hypothesise that it may be Bandai Namco trying to get rid of competition, for an imminent announcement of a Dark Souls HD collection or remake for new-gen consoles - and of course PC.
Some users find Dark Souls to be unplayable without it, whereas some just find it improves the gameplay. Many people claim to have bought the PC version of the game strictly due to DSFix, while others say they will now definitely not be buying it without being able to use the patch.